[PATCH RFC] rcu/tree: Use GFP_MEMALLOC for alloc memory to free memory pattern

From: "Joel Fernandes (Google)" <joel@joelfernandes.org>
To: linux-kernel@vger.kernel.org
Cc: "Joel Fernandes (Google)" <joel@joelfernandes.org>,
	linux-mm@kvack.org, rcu@vger.kernel.org, willy@infradead.org,
	peterz@infradead.org, neilb@suse.com, vbabka@suse.cz,
	mgorman@suse.de, Andrew Morton <akpm@linux-foundation.org>,
	Josh Triplett <josh@joshtriplett.org>,
	Lai Jiangshan <jiangshanlai@gmail.com>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	Steven Rostedt <rostedt@goodmis.org>
Subject: [PATCH RFC] rcu/tree: Use GFP_MEMALLOC for alloc memory to free memory pattern
Date: Tue, 31 Mar 2020 09:16:28 -0400	[thread overview]
Message-ID: <20200331131628.153118-1-joel@joelfernandes.org> (raw)

In kfree_rcu() headless implementation (where the caller need not pass
an rcu_head, but rather directly pass a pointer to an object), we have
a fall-back where we allocate a rcu_head wrapper for the caller (not the
common case). This brings the pattern of needing to allocate some memory
to free some memory.  Currently we use GFP_ATOMIC flag to try harder for
this allocation, however the GFP_MEMALLOC flag is more tailored to this
pattern. We need to try harder not only during atomic context, but also
during non-atomic context anyway. So use the GFP_MEMALLOC flag instead.

Also remove the __GFP_NOWARN flag simply because although we do have a
synchronize_rcu() fallback for absolutely worst case, we still would
like to not enter that path and atleast trigger a warning to the user.

Cc: linux-mm@kvack.org
Cc: rcu@vger.kernel.org
Cc: willy@infradead.org
Cc: peterz@infradead.org
Cc: neilb@suse.com
Cc: vbabka@suse.cz
Cc: mgorman@suse.de
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---

This patch is based on the (not yet upstream) code in:
git://git.kernel.org/pub/scm/linux/kernel/git/jfern/linux.git (branch rcu/kfree)

It is a follow-up to the posted series:
https://lore.kernel.org/lkml/20200330023248.164994-1-joel@joelfernandes.org/


 kernel/rcu/tree.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 4be763355c9fb..965deefffdd58 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3149,7 +3149,7 @@ static inline struct rcu_head *attach_rcu_head_to_object(void *obj)
 
 	if (!ptr)
 		ptr = kmalloc(sizeof(unsigned long *) +
-				sizeof(struct rcu_head), GFP_ATOMIC | __GFP_NOWARN);
+				sizeof(struct rcu_head), GFP_MEMALLOC);
 
 	if (!ptr)
 		return NULL;
-- 
2.26.0.rc2.310.g2932bb562d-goog
